Grape sulfur treatment

The acids present in a given wine are determined by the grape variety, climate, presence of gray rot [Botrytis cinerea), yeasts, bacteria, and various treatments to which the wine may be subjected (sulfur dioxide, ascorbic acid, acidification, desacidifica-tion). There are at least 50 different acids in wine ranging in concentration from 1 or 2gl (tartaric, malic, succinic acids) to hundreds of mgl (citric, lactic, acetic acids) to tens of mgl (pyruvic, shikimic acids). However, tartaric, malic (depending on the MLF), acetic, and succinic acids constitute 80-90% of total complement of wine acids. 

Total SulfuT Dioxide After Alkali Treatment The fact that neutral sodium sulfite does not combine with carbonyl compounds and that the hydroxysulfonic acid compounds are rapidly decomposed on treatment with alkali was used by Ripper (1892) as the basis for the determination of total sulfur dioxide in wine by direct iodine titration. In his method, 50 ml. of wine were pipetted into a 200-ml. flask containing 25 ml. of 1 iV KOH. The mixture was shaken and allowed to stand for 10 to 15 minutes. Then 10 ml. of dilute sulfuric acid (1 + 3) were added, and the solution titrated rapidly with 0.02 N iodine solution to a starch end point which persisted for some time. This method was used as the ofiicial direct titration method for wine in the first edition (1919) of the A.O.A.C. Methods of Analysis in the third (1930) edition it was extended to white grape juice, wine, and similar products (1N NaOH or KOH was used and the solution during standing for 15 minutes was occasionally agitated) hut it was dropped from the fourth (1935) and succeeding editions. Ripper compared his method with the Haas distillation method on ten wines whose SO2 content varied from 42 to 1488 mg. per liter and found the difference between the two to vary from 0 to 5 mg. 

Grape proteases are acidic, with an optimum pH near 2.0. In the pH range of must, 40-60% of the potential proteasic activity exists. Protein hydrolysis activity during the pre-fermentation phase varies greatly, depending on grape maturity and harvest treatments. This certainly affects fermentation kinetics but the relationship has never been established. A slight sulfur dioxide additiou (around 25 mg/1), however, has been confirmed to stimulate proteasic activity. This explains, at least partially, its activation effect on fermentation (Section 8.7.3). 

Peroxidases have long been proven to exist in grapes (Poux and Oumac, 1972). This enzymatic activity is essentially located in grape cell vacuoles. It most likely plays an important role in the oxidative metabolism of phenolic compounds during maturation (Calderon et al, 1992). During prefermentation treatments, the activity of this enzyme seems to be limited by a peroxide deficiency. A low sulfur dioxide concentration is sufficient to destroy these peroxidases. 

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